The present invention relates to a filtering procedure in which the sludge to be filtered is passed into a filtering space between two opposite filter cloths or equivalent of a box filter, and in which the cake of solid matter produced after the separation of fluid is dried by passing a stream of air through it, whereupon the cake is removed from the filtering space.
Box filters are industrially used for the separation of solid materials from fluids. Such materials include e.g. cement, mineral coal and ore concentrates. Box filters are also used for the separation of solid matter from the sludge in sewage disposal plants.
The box filter consists of a number of vertical filter plates forming a block inside which the filtering spaces are enclosed between adjacent plates, each filtering space being delimited by a filter cloth at least on one side of the space. The sludge to be filtered is supplied into the filtering space, where, in order to separate the filtrate, it is subjected to a pressure applied by a pressure medium, e.g. air, supplied into a space behind a separate pressing membrane. After this, the sludge cake thus produced can, if necessary, be washed by means of a washing fluid which is supplied into the filtering space and removed by applying a pressure again. The sludge cake can also be dried by blowing a stream of air, preferably compressed air, through the filtering space. Finally, on completion of the desired treatment of the sludge cake, the filtering space is opened for removal of the cake.
A problem in the use of box filters is that the cake of solid matter formed in the filtering space tends to stick fast to the filter cloth so that it does not fall down from the filtering space when the filter is opened. For this reason, the filtering spaces are opened separately and the filter plates are manually shaken to detach the cake. However, this means that removing the cake takes up a considerable portion of the total time of the filtering cycle. In addition, the procedure requires one person to take care that the cakes are removed from the filter.
Automatic systems for cake removal have also been developed. Their operation is based on the application of vibration to the filter cloths or on the use of detaching instruments, e.g. scrapers or nozzles for compressed air. In these systems, too, generally only one filtering space is treated at a time, but an earlier FI patent application 875030 filed by the present applicant proposes an apparatus for removal of the sludge cake using scrapers and jets of compressed air, which allows simultaneous treatment of all the filtering spaces.
Due to the problems associated with the detaching of the sludge cake from the filter cloth, the filter cloth has in some cases been chosen in the first place with a view to its properties regarding detachment of the cake. However, a cloth thus selected is seldom the best possible choice in respect of its filtering properties.